Mosquito-borne diseases are spreading all over the world at an amazing speed. In recent years, there are more and more reports about mosquito-borne diseases[18–20]. Based on morphological and molecular identification, this study revealed the composition and density of mosquito vectors and other relevant information in Linzhi, Xizang. Using light traps, it is found that the dominant mosquito species in Linzhi were Culex pipiens pallens, followed by Armigeres subalbalus, Aedes vexans, Culex orientalis and Anopheles. Culex pipiens pallens distributed in areas north of the Yangtse River, which is mainly distributed in the area below 3000 m above sea level , The highest elevation at which Culex pipiens pallens has been observed in China is 2,900 metres, in Mainling County, Nyingchi area, Tibet, which is basically consistent with the findings of this study. The female mosquito of Armigeres subalbalus can stab and suck human blood fiercely. It is the mosquito family with greater harassment. Previous studies in Europe have demonstrated that Aedes vexans is not an effective vector of chikungunya virus but may play a secondary role in temperate regions where the mosquito is abundant. It is difficult to distinguish between Culex orientalis and Culex mimulus in morphology. The middle white ring of the beak is obvious which mainly distributed in Heilongjiang, Jilin and Liaoning in China, South Korea, Japan and other places abroad. In South Korea, it has been found that Culex orientalis can carry epidemic encephalitis V virus, which can be viewed as the second vector to transmit Japanese encephalitis except Culex tritaeniorhynchus. It was found that Bayi District had the highest mosquito density in the survey (42.96 mosquitoes per trap per night), Bayi District was located in the North Bank of the Yarlung Zangbo River in the south of Linzhi, Xizang, with sufficient water resources and wide wetland coverage area. Although the altitude was 2985 m, the vegetation coverage was good, and the population density of Bayi District is high. The distribution and density of mosquito species are closely related to the natural environment and human factors, and more mosquito breeding places will be formed due to frequent human activities. The distribution of Culex orientalis was found for the first time in Chayu and Bomi Counties. The density value of Chayu (3.77 mosquitoes per trap per night) is much higher than Bomi County (0.25 mosquitoes per trap per night). It is closely related to the climatic conditions of Chayu County. It has a low altitude area with high temperature, high precipitation and short sunshine time, which is suitable for mosquito breeding. Anopheles were found little in the survey, which were distributed in Chayu County, Xiachayu Town, Milin County, Bomi County and Lang County, it might be related to missing the local maximum temperature in the investigation season, and the other possible reason was that the cowshed preferred by Anopheles was in the vacant period. Compared with the collection of adult mosquitoes in 2009, the species of mosquito vectors are basically the same, but the diversity is lower than that in Motuo County. Due to the investigation of Japanese encephalitis vector in Linzhi, the selection of mosquito catching sites focused on the pig pen and outdoor environment suitable for the survival of Culex tritaeniorhynchus, resulting in the dominant species were Culex pipiens pallens in Linzhi different with Motuo (Culex tritaeniorhynchus).
Using human landing catches, Aedes albopictus was first found in Chayu County xiachayu town and only caught in this area with the high density (26.33 mosquitoes per hour per person). Xiachayu town has low altitude, mild climate and high rainfall, which is suitable for the breeding of Aedes albopictus. Combined with the discovery of Aedes albopictus in Motuo in 2009, it is preliminarily speculated that there is a high potential epidemic risk of dengue fever in Chayu and Motuo County. Some research results show that seasonal weather change, vegetation height, population and land coverage will affect the population and distribution of Aedes albopictus, which are indicators to assess the risk of dengue transmission[25, 26].
The container survey only collected larva in Bomi, Milin and Lang Counties. The results show that the preferred breeding place for mosquitoes is waste tires, cement cans and barrels, which are closely related to human beings. Old tires are often found along the main roads or waste factories, where they are usually unattended and exposed to rain and sunshine for a long time. In communities with intermittent or inadequate water supply, cement tanks and buckets are the most common water storage containers. Although Aedes albopictus preferred water, no Aedes albopictus was found in this kind of habitat. It can be preliminarily inferred that Aedes albopictus did not occur population diffusion in Linzhi.
Aedes albopictus and Culex orientalis were found for the first time and were new records. In South Korea, Japan and other places, Culex orientalis is considered as an important vector for the transmission of Japanese encephalitis except Culex tritaeniorhynchus. It can not be considered that there is no epidemic risk of Japanese encephalitis in this area, so further virological testing is needed for certification. It has been reported to explore the correlation between malaria and meteorological factors in Motuo County, Xizang. The results show that temperature, relative humidity, rainfall and other meteorological variables are important environmental factors for malaria transmission. It can be preliminarily inferred that with the progress of malaria elimination, the local malaria epidemic risk has been at a very low risk level. Climate change, population flow and environmental change play an important role in mosquito distribution in Linzhi. In 2009, Aedes albopictus was first found in Motuo County, while in this study, Aedes albopictus was only found in Xiachayu. It is preliminarily speculated that there is mosquito diffusion barrier in Linzhi. Therefore, in order to better understand the local ecological characteristics of Aedes albopictus, it is urgent to further explore the possibility of population diffusion of Aedes albopictus in Linzhi. In the future, further warming is expected, and further economic development in Linzhi will lead to even greater movement of freight and people. These conditions raise the risk of outbreak of mosquito-borne diseases in a population with no prior exposure to such infections[29, 30]. Therefore, it is urgent to strengthen the detection and monitoring of mosquito-borne diseases.
This study has limitations due to time and traffic restrictions, the research sites did not cover the whole area of Linzhi, but the six counties selected can fully represent the mosquito surveillance results of Linzhi. However, due to the maintenance of Motuo road traffic, the epidemic risk of mosquito diseases in Motuo County can only be preliminarily speculated. Secondly, the survey was carried out in August and September 2019 when the temperature is between 10℃-26℃ slightly lower than the peak of mosquito activity. However, the results of this study can be used to preliminarily explore the species and distribution of mosquitoes in Linzhi, so as to speculate the epidemic risk of mosquito-borne diseases. The agricultural area along the border between China and Myanmar is rich in mosquitoes with obvious seasonality. The composition and density of mosquito vectors are greatly affected by the natural environment, and the risk of mosquito-borne disease transmission is relatively high. As a neighboring country of China, India has a wide range of mosquito species, and the prevalence of dengue fever, chikungunya fever, malaria and other mosquito borne diseases is serious[32, 33]. Therefore, it is speculated that the mosquito density will increase in high temperature season, the diversity of mosquito species will increase, and the potential epidemic risk of various diseases will increase slightly. It is suggested that mosquito surveillance should be carried out again in Linzhi during the peak period of mosquito activity and the opening of Motuo road to obtain more accurate mosquito distribution and epidemic risk assessment of mosquito diseases. Mosquito monitoring is of great significance for the assessment of mosquito-borne diseases, which is consistent with the results of most studies.
Mosquitoes had heterogeneous distribution throughout Linzhi, Xizang. With the increasingly developed traffic and global warming, it is necessary to further determine the composition and density of mosquitoes in this area, so as to assess the risk of disease transmission and establish an effective mosquito surveillance system. With the continuous progress and development of Xizang's economy, people pay more attention to the pursuit of health. Mosquitoes can breed and breed in the high-altitude urban environment of Xizang, so that people have the risk of disease. Strengthened community health education and engagement should be conducted to better guarantee the health and life safety of local citizens. Mosquito-borne viruses pose a serious health threat to people. Mosquito surveillance is an essential part to control mosquito-borne diseases, and sometimes it is the only effective way to block or reduce the spread of these diseases (such as dengue fever)[36, 37]. Using effective monitoring tools to characterize the species composition and density of mosquitoes is the most important step to control and assess the risk of mosquito-borne diseases outbreak[28, 38]. It is suggested that Aedes vector monitoring should be carried out again in the high-risk counties in order to better grasp the species, distribution and growth and decline of mosquito vectors in the area, and increase the number and frequency of monitoring appropriately through monitoring data, so as to provide theoretical basis for scientific mosquito control and control.